Possibly?

While that seems quite possible, it is not going to happen, probably. Since it may not be "one gene makes chemical" but maybe the chemical is a byproduct to a process the plant undergoes during maturation. Scientists have managed to synthesize THC and even put it into pill form (Can't find the article now), but there are still other cannabonoids that are active in the plant causing the "getting high" function. I don't think the plausible or cheapest by any fashion (nor would any high quality SCIENCE be really done with a project like this one. It would be more of a plug-and-chug), since those other cannabonoids would have to be either synthesized, identified, or extracted in order to make such a plan work.

Right now, legalization of the plant would go a long way toward your original goal, since research on the subject would skyrocket.